Mitogen-activated protein kinases (MAPK) are a conserved family of enzymes that relay and propagate external stimuli, using phosphorylation cascades to generate a coordinated cellular response to the environment. The MAPK are proline-directed serine/threonine-specific protein kinases that regulate cellular activities, such as gene expression, mitosis, differentiation, and cell survival/apoptosis. To date, 4 distinct classes of mammalian MAPK have been identified: the extracellular signaling kinases (ERK1 and 2), the c-jun N-terminal kinase-1 (JNK1-3), the p38 MAPK (p38α, β, γ, and δ), and ERK5. The MAPK are activated by the dual phosphorylation of Thr and Tyr residues within a TXY activation motif by coordinated dual-specificity MAPKK, where X is Glu, Pro, and Gly in ERK, JNK, and p38 MAPK, respectively. MAPK are 60-70% identical to each other, yet differ in their activation loop sequences and sizes. The activation loop is adjacent to the enzyme-active site, and its phosphorylation allows the enzyme to reposition active-site residues into the optimal orientation for substrate binding and catalysis. Downstream substrates of MAPK include mitogen-activated protein-kinase-activated protein (MAPKAP) kinases and transcription factors, the phosphorylation of which, either directly or indirectly, regulates gene expression at several points, including transcription, nuclear export, and mRNA stability and translation. The cellular consequences of MAPK activation include inflammation, apoptosis, differentiation, and proliferation.
Distinct genes encode 4 p38 MAPK in humans: p38α, β, γ, and δ. Significant amino acid sequence homology is observed among the 4 isoforms, with 60%-75% overall sequence identity and >90% identity within the kinase domains. Tissue-selective expression is observed, with p38γ found predominantly in skeletal muscle, p38δ in the testes, pancreas, and small intestine. In contrast, p38α and β are more ubiquitously expressed.
An understanding of the broad biologic and pathophysiological roles of p38 MAPK family members has grown significantly over the past decade, as has the complexity of the signaling network leading to their activation. Scientific exploration of this pathway from biological, cellular, and in vivo perspectives was largely enabled by the availability of well-behaved, selective, small-molecule inhibitors of p38 MAPK that target the α and, to a lesser extent, β isoforms. p38α MAPK is the major isoform involved in the immune and inflammatory response. As such its function is critical for the production and activity of multiple pro-inflammatory cytokines, including TNFα, IL-1, IL-6, and IL-8, in cells such as macrophages, monocytes, synovial cells, and endothelial cells. p38 MAPK is also responsible for the induction of key inflammatory enzymes such as COX2 and iNOS, the major sources of eicosanoids and nitric oxide at sites of inflammation, respectively. Additionally, the p38 MAPK pathway regulates the expression of matrix metalloproteinases (MMP), including MMP2, MMP9, and MMP13.
The use of selective and potent inhibitors has facilitated the discovery of several families of p38 MAPK substrates, including transcription factors, MAPKAP kinases, and other enzymes. p38 MAPK can directly phosphorylate several transcription factors, such as myocyte-specific enhancer binding factor 2C (MEF2C), CHOP, peroxisome proliferator-activated receptor (PPAR) α, PPAR γ co-activator 1 and p53. These transcription factors are involved in cellular functions such as apoptosis, gluconeogenesis, and synthesis of enzymes involved in fatty acid oxidation. p38 MAPK is also involved in the direct or indirect phosphorylation of enzyme substrates, such as cytosolic phospholipase A2, and the Cdc25 phosphatases, which are involved in the activation of cyclin-dependent protein kinase activity and cell-cycle regulation. Therefore in addition to its role in the inflammatory response, p38 MAPK has other functions associated with normal and abnormal cell growth and survival as well as cellular function and homeostasis.
The MAPKAP kinases—MK2, MK-3, and PRAK—are selectively phosphorylated by p38 MAPK, while the phosphorylation of MSK1/2, MNK1/2, and RSKb is catalyzed by both p38 MAPK and ERK. Activation of RSKb is thought to play a role in cell survival, although the identification of substrates has been difficult, due to the lack of specific inhibitors. MNK is involved in the phosphorylation of eukaryotic initiation factor-4E, which binds to the ‘cap’ structure of mRNA and enhances protein translation. MNK phosphorylates the mRNA binding protein hnRNP-A0, a protein that regulates mRNA stability of transcripts encoding inflammatory proteins. MSK1/2 is involved in the phosphorylation of the transcription factors CREB and ATF-1, which regulate AP-1 binding proteins. In addition, MSK1/2 can phosphorylate Histone H3, which is involved in chromatin remodeling. While evidence suggests that MSK and MNK play a role in the mediation of pro-inflammatory cytokines, in vivo data with selective inhibitors and/or knockout mice are lacking.
MK-2, MK-3, and PRAK, once phosphorylated and activated by p38 MAPK, share similar substrate specificities. All of these kinases can phosphorylate the small heat-shock protein Hsp27. Studies have shown that the PRAK- and MK3-deficient mice do not display any resistance to endotoxic shock or a decrease in lipopolysaccharide-(LPS)-induced cytokine production. In contrast, MK-2-deficient mice show a resistance to endotoxic shock and an impaired inflammatory response, as well as a significantly decreased production of cytokines such as TNFα, IFNγ and IL-6. Thus, the p38/MK2 axis specifically is necessary and sufficient for mediating pro-inflammatory responses.
Recently, Davidson et al (2004) Discovery and characterization of a substrate selective p38alpha inhibitor, Biochemistry 43:11658-71, described a novel approach for increasing selectivity of a p38 MAPK inhibitors. In these studies, a high throughput screen was carried out using an assay that measured the p38-dependent phosphorylation and activation of MK2. The p38:MK2 complex is very stable with a Kd of 6 nM. The binding affinity of p38 for MK2 is driven by the C-terminal domain of MK2 containing several positively charged amino acid residues. Crystallographic studies of the p38:MK2 complex demonstrated that the C-terminal region of MK2 wraps around p38α and binds to the negatively charged ED binding site. The tight binding of p38 to MK2 may give rise to conformational changes providing additional binding pockets for inhibitors that would specifically be dependent upon the p38:MK2 interaction.
Taking advantage of the p38:MK2 interaction and using MK2 as the p38 substrate, a novel inhibitor of p38α was discovered exhibiting interesting properties. This inhibitor demonstrated substrate selectivity by preventing the p38α dependent phosphorylation of MK2 (Ki app 300 nM) while sparing the p38α dependent phosphorylation of ATF2 (Ki app>20 uM). This novel inhibitor is functionally unique compared with traditional p38 ATP competitive inhibitors that block the p38-dependent phosphorylation of all p38 substrates. A second independent study also describes p38 inhibitors with unique mechanistic properties. This work demonstrates a novel mechanism for the selective inhibition of the p38 dependent phosphorylation of MK2. Unlike the previous study of Davidson et al., these mechanistically unique compounds are competitive with ATP and stabilize the p38/MK2 complex. Taken together, these two studies clearly prove the concept that selective p38/MK2 axis blockade is achievable with small molecule inhibitors. In comparison to traditional p38 MAPK inhibitors these p38/MK2 inhibitors should retain or enhance potency and exhibit improved safety features in animal models of disease or in human clinical settings.
The p38/MK2 role in the regulation of inflammatory cytokines (TNFα, IL-1β, IL-6) and enzymes responsible for inflammation (COX-2, iNOS, and MMPs) makes it an attractive drug target. Several classical p38 MAPK inhibitors have progressed to testing in clinical trials. Some of these candidates have failed, for safety or other reasons, but several have reported clinical data in diseases such as rheumatoid arthritis, pain, Crohn's disease, acute coronary syndrome, multiple myeloma and chronic obstructive pulmonary disease. In addition to these diseases several IL-1β mediated diseases could be impacted by a p38 inhibitor based upon the key role for the p38 MAPK pathway in the biosynthesis and activity of this cytokine. These diseases include the family of cryopyrin associated periodic disorders (CAPS), chronic gout, diabetes, Still's disease, Familial Mediterranean Fever among others.
In addition to human inflammatory pathways, p38 MAPK has been linked to canine B cell growth and survival. The role of p38 MAPK in B cell growth suggests that inhibition of this enzyme may be therapeutically beneficial for the treatment of canine B cell lymphoma. Canine lymphoma is one of the most common malignancies diagnosed in companion animals representing 10-25% of canine neoplasms and >80% of the hematopoietic tumors. An orally available, selective B cell growth inhibitor would meet a significant unmet medical need.
Compounds useful for treating diseases and conditions caused or exacerbated by unregulated p38 MAP Kinase and/or TNF activity are described in WO 2000/017175 published 30 Mar. 2000. The compounds described therein include a class of substituted urea compounds.
Compounds useful for treating diseases and conditions caused or exacerbated by unregulated p38 MAP Kinase and/or TNF activity are described in WO 2000/071535 published 30 Nov. 2000. The compounds described therein include a class of indole-type compounds.
Compounds useful for treating diseases and conditions caused or exacerbated by unregulated p38 MAP Kinase and/or TNF activity are described in WO 2002/042292 published 30 May 2002. The compounds described therein include a class of coupled indole-type derivatives.
Compounds useful for prophylaxis or treatment of circulatory diseases, metabolic diseases and/or central nervous system diseases are described in WO 2008/062905 published 29 May 2008. The compounds described therein include an alkyl-pyrimidinone-phenyl compounds wherein the phenyl fragment is substituted with a cyclopropyl radical, e.g., 6-butyl-3-(3-cyclopropylphenyl)-2methyl-5-{[2′-(5-oxo-4,5-dihydro-1,2,4-oxadizol-3-yl)biphenyl-4-yl]methyl}pyrimidin-4(3H)-one.
Various potential inhibitors or modulators of p38 kinase and the p38 kinase pathway are described in WO 2005/018557 published 3 Mar. 2005. The compounds described therein include di-fluorophenyl-methoxy-pyridinone-pyridyl compounds wherein the pyridyl fragment is substituted with various radicals including alkyl, alkenyl, hydroxyalkyl, halo, cyano, amino, carboxy, carbamoyl, methoxycarbonyl and hydroxyalkenylimino radicals.
Compounds useful for treating diseases and conditions caused or exacerbated by unregulated p38 MAP Kinase and/or TNF activity are described in US 2007/0167621 published 19 Jul. 2007. The compounds described therein include di-fluorophenyl-methoxy-pyrimidinone-phenyl compounds wherein the phenyl fragment is substituted with methyl amido radical.
Compounds useful for treating diseases and conditions caused or exacerbated by unregulated p38 MAP Kinase and/or TNF activity are described in WO 2004/087677 published 14 Oct. 2004. The compounds described therein include di-fluorophenyl-methoxy-pyrimidinone-phenyl compounds wherein the phenyl fragment is substituted with piperazinyl or a morpholinyl radical through a carbonyl bridge.
Pyrimidinone derivatives (as inhibitors of protein kinases and useful in treating disorders related to abnormal protein kinase activities such as inflammatory diseases and certain types of cancer), are described in WO 2007/081901 published 19 Jul. 2008. The compounds described therein include di-fluorophenyl-methoxy-pyrimidinone-phenyl compounds wherein the phenyl fragment is substituted with a cyclopropanyl or a morpholinyl radical through an amidoalkylamido bridge.
Pyrimidinone derivatives (as inhibitors of protein kinases and useful in treating disorders related to abnormal protein kinase activities such as inflammatory diseases and certain types of cancer) are described in WO 2008/153942 published 18 Dec. 2008. The compounds described therein include di-fluorophenyl-methoxy-pyrimidinone-phenyl compounds where the phenyl radical is substituted with cyclopentyl or a cyclohexyl radical through an amido bridge.
Compounds useful for treating diseases and conditions caused or exacerbated by unregulated p38 MAP Kinase and/or TNF activity are described in U.S. Pat. No. 7,067,540 published 27 Jun. 2007. The compounds described therein include di-fluorophenyl-methoxy-pyridinone-phenyl compounds wherein the phenyl radical is substituted with a C5-heteroaryl radical (e.g., pyrazolyl or imidazolyl).